300b Push-Pull Amp

I am wondering if there are any person in the forumwith a 2A3 or 300b push-pull amp set-up, that can share their experience with me.  Better, if you are in Klang Valley, can I arrange for an audition?  I am contactable via 012 2236 517 or mengtaihoi@gmail.com

After building several SE amps using triode power tubes (2A3, 45, 300b, 845) one of the diy forumers remarked that push-pull amps is the "way" to-go.  Key points was that (i) all the early schematics for commercial use are based on PP designs, and (ii) SE are for radios. As I understand, PP seems to be the vogue in the early tube years until SE gained popularity in the 1960s, expounded by the European and Japanese.    

I have not heard a triode PP amp, just pentodes (KT88, EL 34 -some may be wired as triodes(?)).  And then, it was just a cursory audition. 

Reg.

2A3 PP amps seems to be quite a rarity.  Chew, one of the forumers, has a 2A3 PP amp but is in storage.

I have decided to build a pair of C3g/2A3 PP monoblocks over the  year-end holidays.  And I have been accumulating parts from various sources.  

Dixon was very generous to sell me his extra pair of Lundahl LL 1660 interstage and a pair of Jensen 220uf PSU caps at a discount.

Wikin traded me a pair of Lundahl LL 1682 PP OPT sometime back - I got that out of storage.

Kakibuku supplied me a pair of aluminium chassis for the build.

I got most of my other parts from ebay, parts connection, JJyAudio - China, etc.

All I am awaiting now is a set of power transformers, filament transformers and chokes from Nicholas Chua - Prometheus Audio.  Delivery is expected on Friday.  I guess I can start the build over the weekend.

I would use minimal parts for the circuit.  



Anyone keen to do a joint build?

Hi Mthoi, 

I think both pp and se has its strength and weakness. SE amp need very clean power supply to be at its best while PP amp has better power supply noise rejection. If not going to drive low sensitivity speaker, SE will always be my first choice. 

I like ur idea of interstage trans without phase spliter design on the PP amp.

I finally received the parts to start the build.  Nicholas - Prometheus Audio delivered the irons today.  The general layout of the monoblocks is in the pic.

I think a PP triode amp sound signature will suit your taste and setup better, MT Hoi. I'm making this judgement based on my recent experience with an 845PP which I heard recently. The musicality of triodes are still apparent which makes acoustic instruments sound real, but the sound structure takes on a more forward and ear poking experience which mimics the dynamics of a real life instrument.

Cheers

mthoi,

i noticed that you didn't include anything to balance the 2A3 power tube current? since both tubes need to work in tandem, current off balance will impact the sound severely where weak bass and early distortion are signs of it.

sound wise, sometimes it depends on the design and components matching. i've heard: single ended triodes that sounded like push pull and vice versa, or tube amps that sound like ss amp and vice versa.

just   my 2 cents.

ken

Hi Ken, Good point raised. 

Using a matched pair of power tubes (and driver tubes - if in PP mode) are important to reduce the distortion in  PP set-up.  I also believe that operating in fixed bias, and lower(ed) operating points, good interstage transformers and good PP OPT also helps to reduce distortion. 

I have since redesigned the schematics to a c3m/300b PP. Over the year-end holidays, I managed to complete the 2 monoblocks, and are currently undergoing burn-in.   

I do agree that you can tune tube amps to your taste. The sonic characteristics of the driver tube do contribute significantly, plus driver tubes, operating points, schematics, etc.  

After a brief listening to the 300b PP, I think it has "potential" - especially when I listened to orchestral and (fast tempo) jazz.  At one point, I wondered, "why did I wait so long to build a triode PP amp?"

Some pics would follow. Further comments after I have 50+hr burn-in (currently only 8 hr).

hi mthoi,

cool. let us know how it goes.

i'm planning for my next amp - pentode PP amp too. perhaps i should try with DHTs first using the same PP OPT.

ken

A pic of internal wiring for one of the channels. I used some low-cost cement resistors to determine the voltage operating points - they would be replaced in the near future.  

The final layout of the monoblocks.  I don't have a quad of 300b tubes - hence one side is using a matched Shuguang treasures 300b and the other uses JJ 300b.  

I changed the layout to include 2 obbligato film oil caps.

The Lundahl LL1660 interstage and LL1682 PP OPT are potted in aluminium casings.  The interstage are wired 4.5:1.  

The chokes, power and filament transformers are from Nicholas Chua - Prometheus Audio. The filament transformers are very good - the two different 2.5v- 0 - 2.5v supply for the pair of 300b tubes has a variance of 0.01v AC! And for both pair of filament transformers!

After wiring, the hum level is negligible.  And I am using AC filament for the c3m and 300b pair.  Good work Nick - worth the wait.

And the 2 pillows behind the speakers are my early attemps to move away from just building amps to start tuning the room.

Nice job! Mthoi.

How's the 300B PP amp sounded compare to your mighty 845 amp?

Its painful to get a match quad of good 300B tubes. They would easily cost more than a pair of 845. Anyhow, it's a good try.

Cheah

Hi Cheah, Tx.  I calculated the 845 delivers 18 watts. The 300b PP probably delivers around 14W in A1 and 28W in A2.  

The 300b PP still retains its 300b triode characteristics but better prat compared to the 845.  

If I can use the analogy, the 12at7/45/845 in SE mode is like a V6 - smooth and powerful.  The c3m/300b in PP is like a turbo engine - fast and powerful.

For vocals and chamber music - the SE wins.  For ochestral, fast jazz - the 300b pp performed better to my ears.  I have not tried any rock music yet.

Hi MTHoi,
   Nice analogy on the 300B P-P sonic quality.   
 
    BTW, noticed that there is a different pair of speaker...Mind eleborate? Thanks.

Hi Vt4c - funny you should ask about the small floor stander - it is a Pink Triangle Ventricle.  

I previously used it as a front speakers for my AV set-up.  Earlier, it was a replacement for my Harbeth P3ES in my Quad 606 set-up.

Now it is decor.